In general, types of combustion equipment such as boilers using coal, oil, gas, or other combustible materials, generate exhaust gasses during combustion. Pollutants such as nitrogen oxides (NOx) may be contained in exhaust gas generated by such equipment, and thus it is necessary to develop techniques for protecting the atmosphere from pollutants such as nitrogen oxides (NOx).
Such nitrogen oxides can be reduced by controlling combustion (a pretreatment method) or treating exhaust gas (a post-treatment method).
Most post-treatment methods employ a selective catalyst reduction (SCR) device as a nitrogen oxide decreasing device. For example, in a nitrogen oxide removal method using an SCR device, exhaust gas is mixed with a reducing agent such as ammonia, and the mixture is passed through a catalyst, so as to convert nitrogen oxides (NOx) included in the exhaust gas into nitrogen (N2) and water (H2O) through chemical reactions expressed by Chemical Formulas 1 and 2 below.ChemistryFigure 14NO+4NH3+O2→4N2+6H2O  [Chem.1]ChemistryFigure 22NO2+4NH3+O2→3N2+6H2O  [Chem.2]
Catalysts have active temperature ranges in which the catalysts are effective for catalysis. For this reason, it may be necessary to increase the temperature of exhaust gas to be within a catalyst active temperature range of 310° C. to 420° C.
In some cases, however, the actual operating temperatures of coal-burning boilers are lower than designed temperatures thereof due to the low caloric power of recently-produced coal. Particularly, when operating a boiler in a low-load condition, it may be difficult to maintain the catalyst active temperature range therein. If a catalyst is used for a certain period of time at a temperature lower that the minimum active temperature thereof, the activity and selectivity of the catalyst may be markedly reduced due to ammonium sulfate (known as a poisoning phenomenon).
For example, sulfur dioxide (SO2) and sulfur trioxide (SO3) are generated during the combustion of coal or oil (heavy oil) in a boiler, and sulfur trioxide (SO3) may react with moisture contained in exhaust gas and some ammonia (NH3) sprayed from the front side of a nitrogen oxide decreasing device such as an SCR device, to form ammonium sulfate (NH4HSO4, (NH4)2SO4) as expressed by Chemical Formulas 3 and 4 below.ChemistryFigure 34NH3+SO3+H2O→NH4HSO4  [Chem.3]ChemistryFigure 42NH3+SO3+H2O→(NH4)2SO4  [Chem.4]
Ammonium sulfate may be coated on the surface of a catalyst to decrease the activeness of the catalyst and corrode the rear side of the nitrogen oxide decreasing device or other devices. In addition, ammonium sulfate may clog exhaust gas passages of the SCR device and a heat exchanger such as an air preheater to increase pressure loss within a boiler. This phenomenon may be increased in severity as the temperature of exhaust gas of the boiler decreases.
Therefore, many catalyst manufacturers recommend that the minimum temperature of exhaust gas of a boiler should be 310° C. or higher in the case of using high-sulfur coal or oil (heavy oil) as fuel so as to prevent a poisoning phenomenon.
If a catalyst is poisoned (that is, a poisoning phenomenon occurs), the catalyst is removed from a nitrogen oxide decreasing device and is chemically treated to remove substances causing the poisoning phenomenon. If the catalyst cannot be regenerated by such a method, the catalyst is discarded.
However, such a catalyst regeneration method requires relatively expensive chemicals, and it may require a significant amount of money to treat waste water. Furthermore, since a boiler can't be operated while a catalyst is regenerated, a great deal of loss may be caused.
For this reason, a great deal of research has been conducted into techniques for regenerating a poisoned catalyst during boiler operations without the necessity of removing a catalyst from a nitrogen oxide decreasing device. For example, Korean Patent No.: 10-917667 discloses a method of increasing the temperature of exhaust gas to regenerate a catalyst used in an incinerator.
However, the method disclosed is for powder catalysts and thus is not suitable for catalysts having a honeycomb, a flat plate, or a wave shapes that are most commonly used in nitrogen oxide decreasing devices (such as SCR devices) of boilers. Furthermore, in the related art, during boiler operations, ammonia has to be continuously supplied to an SCR device, and there is a limit on increasing the temperature of the SCR to 300° C. or higher. Therefore, the efficiency of catalyst regeneration may be low.
Korean Patent No.: 10-949432 discloses a technique of supplying high temperature exhaust gas to an SCR device through a bypass connected from a side of a boiler to the SCR device for regenerating a catalyst of the SCR device during boiler operations.
However, the technique disclosed for regenerating a catalyst using high temperature exhaust gas supplied through a bypass may cause non-uniform temperature distribution in the SCR device, and the temperature of the SCR device may not be maintained within a proper temperature range if exhaust gas flows backwards in the bypass or if a sufficient amount of exhaust gas is not supplied through the bypass. In the related art, a gas distributor is used to uniformly distribute high temperature exhaust gas for preventing non-uniform temperature distribution. However, such a gas distributor may increase the interior pressure of a boiler to lower the thermal efficiency thereof, and the gas distributor may be damaged by constant exposure to high temperature exhaust gas.